$unionWith (aggregation stage)（聚合阶段）

Create Sales Reports from the Union of Yearly Data Collections从年度数据集合联盟创建销售报告

The following examples use the $unionWith stage to combine data and return results from multiple collections. In these examples, each collection contains a year of sales data.以下示例使用$unionWith阶段组合数据并返回多个集合的结果。在这些示例中，每个集合都包含一年的销售数据。

Report 1: All Sales by Year and Stores and Items报告1：按年份、门店和商品列出的所有销售额

The following aggregation creates a yearly sales report that lists all sales by quarter and stores. The pipeline uses $unionWith to combine documents from all four collections:以下汇总创建了一份年度销售报告，其中按季度和门店列出了所有销售额。该管道使用$unionWith组合来自所有四个集合的文档：

db.sales_2017.aggregate( [
   { $set: { _id: "2017" } },
   { $unionWith: { coll: "sales_2018", pipeline: [ { $set: { _id: "2018" } } ] } },
   { $unionWith: { coll: "sales_2019", pipeline: [ { $set: { _id: "2019" } } ] } },
   { $unionWith: { coll: "sales_2020", pipeline: [ { $set: { _id: "2020" } } ] } },
   { $sort: { _id: 1, store: 1, item: 1 } }
] )

Specifically, the aggregation pipeline uses:具体来说，聚合管道使用：

• A $set stage to update the _id field to contain the year.一个$set阶段，用于更新_id字段以包含年份。
• A sequence of $unionWith stages to combine all documents from the four collections, each also using the $set stage on its documents.一系列$unionWith阶段，用于组合四个集合中的所有文档，每个集合也在其文档上使用$set阶段。
• A $sort stage to sort by the _id (the year), the store, and item.$sort阶段，按_id（年份）、store和item进行排序。

Pipeline output:管道输出：

{ "_id" : "2017", "store" : "General Store", "item" : "Chocolates", "quantity" : 150 }
{ "_id" : "2017", "store" : "General Store", "item" : "Cookies", "quantity" : 100 }
{ "_id" : "2017", "store" : "General Store", "item" : "Pie", "quantity" : 10 }
{ "_id" : "2017", "store" : "ShopMart", "item" : "Chocolates", "quantity" : 50 }
{ "_id" : "2017", "store" : "ShopMart", "item" : "Cookies", "quantity" : 120 }
{ "_id" : "2017", "store" : "ShopMart", "item" : "Pie", "quantity" : 5 }
{ "_id" : "2018", "store" : "General Store", "item" : "Cheese", "quantity" : 30 }
{ "_id" : "2018", "store" : "General Store", "item" : "Chocolates", "quantity" : 125 }
{ "_id" : "2018", "store" : "General Store", "item" : "Cookies", "quantity" : 200 }
{ "_id" : "2018", "store" : "General Store", "item" : "Pie", "quantity" : 30 }
{ "_id" : "2018", "store" : "ShopMart", "item" : "Cheese", "quantity" : 50 }
{ "_id" : "2018", "store" : "ShopMart", "item" : "Chocolates", "quantity" : 150 }
{ "_id" : "2018", "store" : "ShopMart", "item" : "Cookies", "quantity" : 100 }
{ "_id" : "2018", "store" : "ShopMart", "item" : "Nuts", "quantity" : 100 }
{ "_id" : "2018", "store" : "ShopMart", "item" : "Pie", "quantity" : 25 }
{ "_id" : "2019", "store" : "General Store", "item" : "Cheese", "quantity" : 50 }
{ "_id" : "2019", "store" : "General Store", "item" : "Chocolates", "quantity" : 125 }
{ "_id" : "2019", "store" : "General Store", "item" : "Cookies", "quantity" : 200 }
{ "_id" : "2019", "store" : "General Store", "item" : "Nuts", "quantity" : 80 }
{ "_id" : "2019", "store" : "General Store", "item" : "Pie", "quantity" : 50 }
{ "_id" : "2019", "store" : "ShopMart", "item" : "Cheese", "quantity" : 20 }
{ "_id" : "2019", "store" : "ShopMart", "item" : "Chocolates", "quantity" : 150 }
{ "_id" : "2019", "store" : "ShopMart", "item" : "Cookies", "quantity" : 100 }
{ "_id" : "2019", "store" : "ShopMart", "item" : "Nuts", "quantity" : 30 }
{ "_id" : "2019", "store" : "ShopMart", "item" : "Pie", "quantity" : 75 }
{ "_id" : "2020", "store" : "General Store", "item" : "Cheese", "quantity" : 100 }
{ "_id" : "2020", "store" : "General Store", "item" : "Chocolates", "quantity" : 200 }
{ "_id" : "2020", "store" : "General Store", "item" : "Cookies", "quantity" : 500 }
{ "_id" : "2020", "store" : "General Store", "item" : "Nuts", "quantity" : 100 }
{ "_id" : "2020", "store" : "General Store", "item" : "Pie", "quantity" : 100 }
{ "_id" : "2020", "store" : "ShopMart", "item" : "Cheese", "quantity" : 100 }
{ "_id" : "2020", "store" : "ShopMart", "item" : "Chocolates", "quantity" : 300 }
{ "_id" : "2020", "store" : "ShopMart", "item" : "Cookies", "quantity" : 400 }
{ "_id" : "2020", "store" : "ShopMart", "item" : "Nuts", "quantity" : 200 }
{ "_id" : "2020", "store" : "ShopMart", "item" : "Pie", "quantity" : 100 }

Report 2: Aggregated Sales by Items报告2：按项目汇总的销售额

The following aggregation creates a sales report that lists the sales quantity per item. The pipeline uses $unionWith to combine documents from all four years:以下聚合创建了一个销售报告，其中列出了每个项目的销售数量。该管道使用$unionWith组合所有四年的文档：

db.sales_2017.aggregate( [
   { $unionWith: "sales_2018" },
   { $unionWith: "sales_2019" },
   { $unionWith: "sales_2020" },
   { $group: { _id: "$item", total: { $sum: "$quantity" } } },
   { $sort: { total: -1 } }
] )

• The sequence of $unionWith stages retrieve documents from the specified collections into the pipeline:$unionWith阶段的顺序将文档从指定集合检索到管道中：
• The $group stage groups by the item field and uses $sum to calculate the total sales quantity per item.$group阶段按item字段分组，并使用$sum计算每个item的总销售数量。
• The $sort stage orders the documents by descending total.$sort阶段按total（总数）降序排列文档。

Pipeline output:管道输出：

{ "_id" : "Cookies", "total" : 1720 }
{ "_id" : "Chocolates", "total" : 1250 }
{ "_id" : "Nuts", "total" : 510 }
{ "_id" : "Pie", "total" : 395 }
{ "_id" : "Cheese", "total" : 350 }

Create a Union with Specified Documents使用指定文档创建联盟

You can use $unionWith to perform a union with documents that you specify in the pipeline field. When you specify a $documents stage in the pipeline field, you perform a union with documents that aren't stored in a separate collection.您可以使用$unionWith与您在pipeline字段中指定的文档执行联合。当您在管道字段中指定$documents阶段时，您将与未存储在单独集合中的文档执行联合。

Create a collection cakeFlavors:创建一个集合cakeFlavors：

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

The following $unionWith operation performs a union with documents specified in the pipeline $documents field:以下$unionWith操作与pipeline $documents字段中指定的文档执行联合：

db.cakeFlavors.aggregate( [
   {
      $unionWith: {
         pipeline: [
            {
               $documents: [
                  { _id: 4, flavor: "orange" },
                  { _id: 5, flavor: "vanilla", price: 20 }
               ]
            }
         ]
      }
   }
] )

Output:输出：

[
   { _id: 1, flavor: 'chocolate' },
   { _id: 2, flavor: 'strawberry' },
   { _id: 3, flavor: 'cherry' },
   { _id: 4, flavor: 'orange' },
   { _id: 5, flavor: 'vanilla', price: 20 }
]

Namespaces in Subpipelines子管道中的命名空间

Starting in MongoDB 8.0, namespaces in subpipelines within $lookup and $unionWith are validated to ensure the correct use of from and coll fields:从MongoDB 8.0开始，$lookup和$unionWith子管道中的命名空间经过验证，以确保from和coll字段的正确使用：

• For $lookup, omit the from field if you use a subpipeline with a stage which doesn't require a specified collection. For example, a $documents stage.对于$lookup，如果您使用的子管道具有不需要指定集合的阶段，请省略from字段。例如，$documents阶段。
• Similarly, for $unionWith, omit the coll field.同样，对于$unionWith，省略coll字段。

Unchanged behavior:行为不变：

• For a $lookup that starts with a stage for a collection, for example a $match or $collStats subpipeline, you must include the from field and specify the collection.对于以集合的阶段开始的$lookup，例如$match或$collStats子管道，您必须包含from字段并指定集合。
• Similarly, for $unionWith, include the coll field and specify the collection.同样，对于$unionWith，包含coll字段并指定集合。

The following scenario shows an example.以下场景显示了一个示例。

Create a collection cakeFlavors:创建一个集合cakeFlavors：

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

Starting in MongoDB 8.0, the following example returns an error because it contains an invalid coll field:从MongoDB 8.0开始，以下示例返回错误，因为它包含无效的coll字段：

db.cakeFlavors.aggregate( [ {
   $unionWith: {
      coll: "cakeFlavors",
      pipeline: [ { $documents: [] } ] }
} ] )

In MongoDB versions before 8.0, the previous example runs.在MongoDB 8.0之前的版本中，前面的示例运行。

For an example with a valid coll field, see Duplicate Results.有关具有有效coll字段的示例，请参阅重复结果。